1. Technical Field
The technical field relates generally to hybrid vehicles and, more particularly, to vehicles equipped with at least two prime movers and provides control over the response by the prime movers to requests for changes in power or angular velocity, particularly when in support of a power take-off operation (PTO).
2. Description of the Problem
Hybrid vehicles are generally equipped with at least two prime movers or systems capable of developing mechanical power. One prime mover is typically a thermal engine such as an internal combustion engine, although it is conceivable that a vehicle could be equipped with a gas turbine or a steam engine. This engine relies on combustion of a hydro-carbon fuel. The second prime mover frequently is a dual function system that can both develop mechanical power and can recapture vehicle kinetic energy during braking (regenerative braking). Recaptured energy can be stored in chemical, electrical or mechanical form. Electrical storage batteries exemplify storage of energy in chemical form. Capacitors store electrical energy. Fly wheels, springs and hydraulic accumulators exemplify methods for the storage of mechanical energy. The stored energy can be used directly or, more commonly, can be converted to a form which can be used to develop mechanical energy to propel the vehicle or to support subsidiary vehicle functions, such as PTO. In either case use of a vehicle thermal engine is reduced sparing fuel consumption.
Electric traction motors which can be back driven to generate electricity are a common second prime mover in hybrid vehicles. Electricity generated when back driven during regenerative braking may be used to charge batteries or it may be stored on capacitors. Alternative types of prime movers in common use are hydraulic or pneumatic pumps which can be back driven to build pressure in a hydraulic or pneumatic accumulator. A fly wheel represents combination of prime mover and storage means.
In a parallel type hybrid vehicle using first and second prime movers, either prime mover may be connected to drive the power take-off operation. However, the mechanical operating characteristics of the prime movers are likely to differ from one another. For example, at most operating speeds, an unloaded electrical motor will exhibit a greater capacity for angular acceleration than will a diesel cycle internal combustion engine. A diesel engine must draw air, compress the air and then convert the heated gas mixture to mechanical energy as the gas expands. The speed at which these events occur is limited. While, spark ignition engines generally respond somewhat more quickly to requests for increased power output than do diesel cycle engines, they suffer the same qualitative restrictions. Gas turbines generally respond less quickly to demands for increased power output than either compression or spark ignition engines due to the need for the exhaust turbine to spool up in speed before more air is delivered to the combustion chamber of the engine.
In contrast, an electric traction motor operates on currents and fields propagating at close to the speed of light. Friction and inertia affect both thermal engines and electric motors, but an electric motor can easily increase its angular velocity 200 to 400% faster than a diesel cycle internal combustion engine. The positive acceleration differences between an electric motor and a gas turbine are likely even greater than between an electric motor and a diesel cycle internal combustion engine. The percentage difference can vary depending upon how fast the respective devices were turning, or what their power output was, before the increase in power demand.
In contrast, any piston engine is likely to respond more quickly to a decrease in power demand than an electric motor or a turbine based engine. This is due to the inherent braking capacity engines of piston based pumps. Generally, under no load conditions, electric motors accelerate much more quickly than internal combustion engines but decelerate more slowly.
The differing capacity for angular acceleration and deceleration of the prime movers can affect the operation of PTO powered equipment. Hydraulic motion control equipment, capstans and the like may rely on a particular rate of change in angular velocity to implement proportional control. An example serves to illustrate this. A hydraulically operated aerial tower or equivalent device used to carry workmen or materials could be configured to operate with switch type devices mounted at an operator station. These are used to increase or decrease the angular velocity of the PTO system's integrated hydraulic pumps. If the primary movers exhibit differing angular acceleration than the PTO system will exhibit differing rates of unloaded acceleration for a given switch selection depending upon which prime mover is operative. The differences will be accentuated if an operation is spread over a shift in operation from one prime mover to the other.